The invention pertains to nuclear magnetic resonance apparatus having a magnet system for generating a homogeneous main magnetic field (H.sub.O), a magnet system for generating gradient fields, a magnet coil for generating a local homogeneous radio-frequency magnetic alternating field in an examination space and a magnet coil for detecting nuclear magnetic resonance signals generated in an object to be examined.
A nuclear magnetic resonance apparatus of this type is described in Computer Tomography, 1, 1981, pp. 2-10, and Chapter 8, page 164 of the book "Nuclear magnetic resonance and its applications to living systems" by D. G. Gadian, Clarendon Press Oxford, 1982 deals with the advantage of using a so-called surface coil for blood stream measurements. Advantages mentioned there are the relatively favourable value of the signal-to-noise ratio that can be achieved during detection, the possibility of accurate measurements in a relatively small region near the coil and the possibility of adapting the shape and size of the coil to the shape and size of an object to be measured. For generating the radio-frequency electromagnetic field, a larger coil is used, often called a body coil, because otherwise the spatial homogeneity in the field is insufficient. If it is desired in such an apparatus for one reason or another to position the transmitting coil and a surface coil as measuring coil other than mutually perpendicular, serious disturbances occur. The surface coil is then activated by the transmitting field to such an extent that the detection circuit becomes overloaded, which may for instance lead to destruction of a pre-amplifier. Conversely,, during transmission the current induced in the surface coil interferes with the radio-frequency transmission signal so that, for example, incorrect 90.degree. and 180.degree. transmitting pulses are produced. These effects result in a severe limitation of the possible applications of surface coils or, on the other hand, many of the advantages of the surface coil are lost due to these disturbances.